Generally speaking, when light passes through a medium, optical phenomena such as incidence, reflection or refraction occur. However, the intensity of laser beam is extremely high and is coherent, and the nonlinear response relevant to the polarization of a material will induce an electric field to light waves when laser wave passes through the material. It might generate harmonic waves from incident light waves at the sum frequency and beat frequency. The effect which is different from linear optical phenomena is called nonlinear optical effects. The crystals having nonlinear optical properties are called nonlinear optical crystals.
One of the most basic and important nonlinear optical effect is the change of the frequency of light. Nonlinear optical crystals can be used to change a fixed frequency of a laser to different frequencies by processes of double frequency, sum frequency, beat frequency or optical parametric amplification. The second harmonic generation generated by doubling frequency of nonlinear optical effects has widespread applications in the application of laser.
Nonlinear optical crystals have very important applications in laser technology. Nonlinear optical crystals can be applied to scientific research, high power lasers, laser medical cosmetology and national defense technology. Only a few nonlinear optical crystals have been successfully commercialized, and thus there is still a need for new nonlinear optical crystals for various applications.